A technology of generating positive ions and negative ions (hereinafter, also referred to as “both ions” or simply as “ions”) in the air has recently been found to have an effect of killing bacteria floating in the air and purify the air. An ion generator employing the technology, such as an air purifier, has matched the comfort and the recent trends towards health-conscious lifestyle, and thus has drawn much attention.
Since ions are invisible, checking generation of ions by direct eye-observation is not possible. Still, users of devices such as air purifiers naturally want to know if ions are successfully generated and if the ions generated have a desired concentration.
In this respect, Patent Literature 1, for example, discloses an air conditioner or air purifier provided with an ion sensor for measuring the ion concentration in the air, and a display for displaying the ion concentration measured with the ion sensor.
An ion sensor of course is preferred to have high precision for precise measurement of the concentration of ions produced in the air.
In this respect, the following sensors are available. Patent Literature 2, for example, discloses a biosensor that changes the voltage to be applied to the back gate to adjust the electric potential of the gate electrode and suppress variation in threshold. Patent Literature 3, for example, discloses a field effect transistor ion sensor which includes a field effect transistor and an ion sensor integrally formed, and reduces the influence of measurement environment.
Also known is an ion generating element provided with an ion sensor portion for determining the amounts of positive ions and negative ions generated from the ion generating portion and a display for displaying the amounts of ions determined as described in, for example, Patent Literature 4. Furthermore, a remote control for electric appliances with a built-in ion sensor is known which is provided with an ion sensor for measuring the ion concentration in the air and a display for displaying the current state of the electric appliances, as described in, for example, Patent Literature 5.